A 23-year-old man presented with the right upper monoparesis. The right little finger paresis was apparent at first, and ring finger two weeks later, and middle, index, thumb were simultaneously four weeks later. Then the monoparesis gradually progressed to the proximal upper limb. Magnetic resonance imaging showed a small lesion at the knob of the left precentral gyrus. The lesion was low-intensity on T1-, and high-intensity on T2-weighted images, and clearly detected on high-intensity on FLAIR images, but showed no enhancement by gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA). Angiography and thallium scintigraphy showed no remarkable findings. Proton MR spectroscopy demonstrated lower N-acetylaspartate (NAA) and higher choline (Cho) level compared to the contralateral cortico-subcortical area. Diffusion weighted images demonstrated low apparent diffusion coefficient (ADC) value and high intensity on b = 1,000. To clarify the diagnosis of the lesion, we performed open biopsy by using the neuronavigation system to detect the lesion accurately and minimize the biopsy. Histological examination revealed an high grade astrocytoma with high MlIB-1 index over 30%. The progressive symptoms were due to highly infiltrative and proliferative nature of the tumor arising in the focal hand area of the primary motor cortex, according to the homunculus. We discuss herein the neuroimagings of the case that was considered to be in the initial stage of a malignant tumor.